U.S. Pat. No. 3,414,459 describes a sheet of absorbent material made by a method known as point-to-point combination using at least two embossed, creped paper sheets. The embossed pattern in the sheets consists of regularly arrayed protuberances having a density of 3 to 30 protuberances per cm.sup.2. The protuberances on each sheet are made by mechanical deformation using a metal cylinder relief engraved with a desired pattern and a smooth rubber cylinder. The protuberances are present on the same side of the plane of each sheet. The height of the protuberances are between 0.2-1 mm. The protrubences take up about 10-60% of the total surface area of the sheet.
In order to provide paper sheets with the flexibility, compressibility and softness required in a paper sheet product useful for sanitary and household applications, mechanical calendaring is applied to a double sheet structure which issues from bonding cylinders during the making of the sheet product so that the thickness of the sheet is reduced by at least 50%. The purpose of this treatment is to lower the compressive modulus of the double sheet structure, i.e. lower the resistance of the sheet to deformation when the sheet is subjected to compression applied perpendicularly to its principal plane. Accordingly, when a user handles the sheet, a greater softness and sponginess will be felt.
As described in the aforementioned patent, two sheets are bonded by passing the sheets through a nip present between two identical, metal embossing cylinders. Such cylinders are positioned in parallel relationship to each other and are driven by belts or other equivalent means in such a manner that their rotational speeds are equal but opposite. The drive means are adjusted so that the protrubences on the cylinders coincide as precisely as possible in the nip.
Implicitly this patent assumes that the metal cylinders are perfectly engraved and that no variation is present between the points of the protrubences. Actually, however, in conventional engraving techniques, two kinds of shifts between the protrubences are possible.
A circumferential shift between the points of the protrubences located on the same theoretical cylinder generatrix can occur. For example, in a cylinder 2.60 m long, a 5/10 mm circumferential shift was measured between the end protrubences on the same generatrix. This shift occurs independently of the pattern density.
Additionally, an axial shift along the same generatrix is possible with an amplitude being a sinusoidal function with a frequency depending on the pattern density of the particular production method. The more dense the pattern, the higher the oscillation frequency along the generatrix. A maximum amplitude of 1/10 mm was measured with respect to the cylinder referred to above.
These manufacturing tolerances do not affect the bonding quality between two sheets of material when the contact surface of the protrubence points is fairly large. The probability is slight in that situation that two protrubence points equally indexed on two cylinders would be so mutually shifted that they would no longer result in bonding. Additionally, when a double sheet structure made by this method is transformed into rolls of narrow working widths, for example, 10, 20 or 30 cm, unbonded sheets are not observed.
However, when this bonding technique is used with previously embossed sheets of creped paper having a fine or dense pattern and comparatively small contact areas, it was found that entire zones constituting strips in the machine direction of advance were not bonded. Accordingly, when these sheets were processed into rolls, such as sanitary paper, having a working width of less than that of the nonbonded strips, cut off products consisting of two unbonded, wound sheets were found to be present within the unbonded strip areas. The waste of material therefor is significant.